The accomplishment of attaining high-temperature superconductivity is of immense scientific and technological importance. Several critical transition temperature barriers have been breached since the long standing record temperature of 23.2 degrees Kelvin for Nb.sub.3 Ge was exceeded. However, the ultimate benchmark of room temperature superconductivity remained an unattainable goal until the instant inventors developed a process for successfully introducing parametrically modifier elements into the aforementioned class of ceramic defect oxide materials so as to fabricate a new class of high critical transition temperature materials.
In late 1986, the superconducting properties of certain defect oxide type materials, which materials are variations of a well-known class of inorganic structures called perovskites, were observed by Bednorz and Mueller. The Bednorz and Mueller work was based upon materials developed by Michel and Raveau. The materials which Bednorz and Mueller observed contained lanthanum, barium, copper, and oxygen, and were reported to be superconducting at a temperature of about 30 degrees Kelvin.
Subsequently, many workers in the field became involved in efforts that resulted in the increase of the critical temperature, T.sub.c (the temperature at which electrons are able to move through a material without encountering any resistance to that motion), by the substitution of yttrium for lanthanum. Upon analysis, the superconducting composition was found to be a perovskite defect oxide of the Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7 type, possible an orthorhombically distorted perovskite. This composition has also been referred to as a tetragonal, square-planar structure (see P. H. Hor, R. L. Meng, Y. Q. Wang, L. Gao, Z. J. Huang, J. Bechtold, K. Forster and C. W. Chu, Superconductivity Above 90 K In the Square Planar Compound System ABa.sub.2 Cu.sub.3 O.sub.6+x With A=La, Nd, Sm, Gd, Gn and Lu, Mar. 17, 1987) with the Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.y defect oxide perovskite phase being responsible for the superconducting properties. Further work with this phase effectively raised the critical temperature to a few degrees above 90 degrees Kelvin (a temperature above the atmospheric boiling point of liquid nitrogen).
However, up to this point in time, scientists in the field had been unable to raise the temperature of superconducting materials to the temperature of the environment, i.e., room temperature, Therefore, prior to the work of the instant inventors at the laboratory at Energy Conversion Devices, Inc., three plateaus in raising the critical temperature of superconducting materials had been broached. The first plateau was represented by the pre-ceramic oxide superconducting materials, which materials were limited to a T.sub.c of about 23-25 degrees Kelvin; the second plateau was represented by the work of Bednorz and Mueller on lanthanum, barium, copper and oxygen systems, which systems resulted in superconducting temperatures of about 30 degrees Kelvin; and the third plateau was represented by the yttrium, barium, copper and oxygen systems, which systems provided for a rise in critical temperature to about 90-95 degrees Kelvin. However, it was not until the development of the new class of parametrically modified ceramic based fluoro-oxide materials by Energy Conversion Devices, Inc. that the critical benchmark of room temperature superconductivity became attainable.
Later workers in the field have attempted the total and/or partial replacement of the yttrium and/or lanthanum with other Group IIIA metals (including Rare Earths), especially with scandium, europium, lutetium, neodymium, praseodymium and gadolinium. The same and other workers in the field have also attempted the total and/or partial replacement of barium with other group IIA metals, such as strontium and calcium.
The defect oxide perovskite phase, having the general composition M.sub.1.sup.IIIA M.sub.2.sup.IIA M.sub.3.sup.IB O.sub.y, was identified by several groups utilizing electron microprobe analysis, x-ray diffraction, scanning electron microscopy, and transmission electron microscopy. These groups have independentally characterized this defect oxide perovskite, M.sub.1.sup.IIIA M.sub.2.sup.IIA M.sub.3.sup.IB O.sub.y phase as having the crystallographic unit cell structure shown in FIG. 1.
The perovskite structure is similar to the naturally occurring calcium titanate structure, CaTiO.sub.3, also shown by other AB.sub.2 O.sub.3 -type oxides having at least one cation much larger than the other cation or cations, including the tungsten bronzes, NaWO.sub.3, strontium titanate, barium titanate, YAlO.sub.3, LaGaO.sub.3, NaNbO.sub.3, KNbO.sub.3, KMgF.sub.3, KNiF.sub.3, and KZnF.sub.3, among others. In the perovskite structure the larger ions (La.sup.+3 =1.15 angstroms, Ba.sup.+2 =1.35 angstroms, and O.sup.+2 =1.40 angstroms, Linus Pauling, The Nature of the Chemical Bond, 3rd Edition, Table 13-3, "Crystal Radii and Univalent Radii of Ions") form a cubic close packed structure, with the smaller ions (Cu.sup.+2 =0.96 angstroms, Y.sup.+3 =0.90 angstroms, Pauling, op. cit.) arranged in occupying octahedral interstices in an ordered pattern. Together they form a cubic close packed (face centered cubic) array.
The superconducting perovskite type materials are ceramic based defect oxides. That is, the superconducting phases of the perovskite type materials are solids in which different kinds of atoms occupy structurally equivalent sites, and where, in order to preserve electrical neutrality, some sites are unoccupied, or vacant. Since these vacancies may be filled with mobile oxygen atoms, only local order is prevelant. It is also clear that these vacant atoms form lattice defects, which defects have, generally, profound affects upon the electrical parameters of the superconducting material, and more particularly upon the oxidation states of the copper atoms in the unit cell thereof.
In order to achieve yet higher critical temperatures, researchers at Energy Conversion Devices, Inc. realized that it would be necessary to develop a superconductivity material in which the chemistry thereof was engineered so as to alter the local chemical and electrical environment. For example, it has been established that the mobility of oxygen atoms in the Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7 ceramic based systems is very high and therefore the location of those mobile oxygen vacancies at any point in time contribute to the presence or absence of high T.sub.c superconducting phases. It is this oxygen mobility and changing local environment which results in the unstable electronic properties of this class of superconducting materials. Although definitive proof is not as yet available, the instant inventors have preliminary evidence that the addition of the very small and highly electronegative fluorine atoms operates to effectively occupy lattice sites in the ceramic based fluoro-oxide class of materials so as to cause "grid lock" and provide an impediment to the mobility of oxygen atoms. The result is a stabilized high critical temperature superconducting material.
The perovskite ceramic defect oxide system shown in FIG. 1 has a recurring structure of (1) a M.sup.IB -O plane of a first type with vacant O sites, (2) a M.sup.IIA -O plane, (3) a M.sup.IB -O plane of a second type with fully occupied O sites, (4) a M.sup.IIIA plane with O sites, (5), another M.sup.IB -O plane of the first type with fully occupied O sites, (6) another plane of the M.sup.IIA -O type, and a (7) a second M.sup.IB -O plane of the first type, with O site vacancies. It may thus be seen that the unit cell (of the superconducting material) so formed has seven substantially parallel planes (i.e., shown as ab planes) spacedly disposed along the substantially parallel to the c axis thereof. It can further be observed in FIG. 1, that the c axis represents the largest spatial dimension of the unit cell, (approximately 11.67 angstroms).
The central plane is a plane of the M.sup.IIIA -O type, as a Y-O or La-O plane, with the Group IIIA metal being surrounded as its four coplanar corners by oxygen sites, which may be occupied or vacant. Immediately above and below this M.sup.IIIA -O plane are equivalent M.sup.IB -O planes of the second type, i.e., Cu-O planes, with the Group IB metal ions being at the four corners of the plane, and occupied oxygen sites being along each edge of the planes. These square planar M.sup.IB atoms (or ions), each surrounded by four oxygen atoms (or ions) have been reported to be critical to superconductivity in the defect oxide perovskites. A pair of M.sup.IIA -O planes, as Ba-O planes lie atop and below these fully occupied first type M.sup.IB -O planes. The M.sup.IIA -O planes, formed with the Group IIA metal, as barium, at the center have fully occupied oxygen sites, with the oxygens disposed directly above and below the Group IB metal sites of the adjacent planes. The M.sup.IIA -O planes are disposed between M.sup.IB -O planes, as shown in FIG. 1, with the first type M.sup.IB -O planes disposed on opposite sides thereof relative to the second type M.sup.IB -O planes. As mentioned above, the deficiencies, that is, the vacancies (unoccupied sites) reported to reside in the first type M.sup.IB -O planes are the result of the requirement of electrical neutrality. While the vacancies are generally reported to be in the M.sup.IB -O planes, they may also be in the other planes, as in the M.sup.IIA -O planes and/or in the M.sup.IIIA -O planes.
While the aforementioned researchers have been responsible for the developments which occurred over the course of the last several months and which effectively raised the critical temperature of the aforementioned classes of superconducting materials up to about 90 degrees Kelvin; it was not until the work of the instant inventors, that the critical temperature of the high T.sub.c phases of said superconducting material have been raised beyond about the 90-95 degree Kelvin plateau. More particularly, the instant inventors have previously provided evidence (in the commonly assigned application referred to in the following paragraph) of superconducting phases in "modified" materials as high as 155 to 168 degrees Kelvin at which temperatures, a zero resistance state has been measured. These materials also presented magnetic indication of the presence of yet higher temperature superconducting phases of said superconducting material. In addition, electrical conduction measurements reveal that the "modified" superconducting materials of the instant inventors achieve a resistance value approximately four times lower than that of single crystal copper before the zero resistance state thereof is reached. This is taken as clear evidence of the presence of some volume fraction of other high transition temperature superconducting phases of the material. It is also worth noting that recent magnetic measurements of the modified superconducting material indicate superconducting phases as high as 370 to 390 degrees Kelvin (the temperature of boiling water).
The inventors of the instant invention, while working with the perovskite, ceramic defect oxide class of superconducting materials, have previously been successful in introducing at least one "parametric modifier" into the unit cell thereof so as to improve the superconducting properties of that class of superconducting materials. This introduction of parametric modifier elements through the use of a "shake and bake" powder metallurgical process was fully disclosed in commonly assigned, copending U.S. application Ser. No. 043,279, filed Apr. 27, 1987 by Stanford R. Ovshinsky and Rosa Young entitled "Parametrically Modified Superconducting Material", the disclosure of which is incorporated herein by reference. The incorporation of the parametric modifier into the superconducting phase resulted in overcoming what had appeared to be a barrier to raising the critical temperatures of superconducting material to additional heights. It was in this way that Ovshinsky, et. al. provided a new mechanism for controllably affecting fundamental parameters of said superconducting materials, which parameters determine the critical temperature thereof. It was also in this manner that the door was opened for further increases in critical temperatures of superconducting materials, even beyond the 155 to 168 degree Kelvin temperatures reported in said Ovshinsky, et. al. application. More specifically, the addition of the parametric modifier, fluorine, to the superconducting material resulted in the identification of a superconducting phase of said defect oxide type superconducting material which has recently been shown to achieve a critical temperature at about 90.degree. F. (a temperature well above room temperature) with indications of the presence of yet higher temperature superconducting phases. It is to be additionally noted that the significance of room temperature superconductivity is more than a quantatative phenomena; said room temperature critical temperature makes possible a host of new applications which were heretofore unattainable.
While the aforementioned introduction of a parametric modifier element (such as fluorine) into a ceramic based defect oxide material successfully raised the critical temperature thereof to above room temperature, which room temperature superconductivity represented a goal long sought after by scientists in the field; recent researchers were not sure of the current carrying capabilities of said high critical temperature phases. However, this doubt has recently been resolved by scientists at Nippon Telegraph and Telephone in Japan who found that said high T.sub.c ceramic defect oxide material could carry current densities of approximately 1.8.times.10.sup.6 amperes per square centimeter by measuring the current carried a single crystal thin film of a Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7 material in the direction of movement parallel to the ab planes, i.e., perpendicular to the c axis of the unit cell thereof. Further, the "1,2,3" crystal was found to be strongly anisotropic and could only carry about 10,000 amperes per square centimeter of current in a direction other than along said ab planes (which capacity of 1.8.times.10.sup.6 amperes per square centimeter was about the same as that of the prior niobium tin class of superconducting materials). Of still further significance is the fact that the highest current density measured in the multiphase Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7 class of superconducting materials, as prepared by conventional powder metallurgical techniques, was about 10,000 amperes per square centimeters, i.e., the limiting current density parallel to the c axis.
The significance of these number is that the high T.sub.c phases of superconducting material of the Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7 class are highly unaligned and therefore the current density measured is limited by those grains thereof showing the highest resistance to current flow, in this case, the unaligned grains in which the current is measured along the c axis rather than along the ab planes. Only by aligning all of the discrete grains of a multi-grained superconducting material would it be possible to achieve the type of current densities which have been shown to exist. Further, epitaxially grown single crystal material is too costly for use in the fabrication of commercial devices and single crystals are inherently brittle and inflexible so that microcrystalline grains must be utilized in order to make it commercially feasible to fabricate wire or other flexible superconducting material. Therefore, it is imperative that the crystallites of such multi-grained ceramic based superconducting material be aligned along the c axes thereof in order to both attain flexible material and achieve high current densities. It is to just this objective that the subject invention is directed.
Further, since zero resistivity measurements on a bulk sample of powder metallurgically produced material has only a very small volume percentage of high T.sub.c superconducting phase material, the superconducting pathway must be through a fortuitously provided filamentary pathway extending between the two points of the probe. And since the axes of discrete grains of the high T.sub.c superconducting phase are randomly oriented, it is of no surprise that researchers had heretofore been unable to measure high current densities. Until such time as the volume formation of the high T.sub.c superconducting phases can be increased, it is essential that the volume fraction which is provided have the unit cells of the grains thereof substantially aligned so that the highest possible current densities can be achieved in the aforementioned filamentary pathways.